Inrush current, switch-on surge, or current spike refers to the current drawn by electric motors when first energized. Current drawn by large, industrial electric motors can be substantial due to the initially stationary rotor. Electric motors can also spontaneously draw large current during operation, due to operating conditions such as a feed jam in an industrial grinder. Electric companies typically levy charges against customers for causing large spikes in current demand, which can be substantial when caused by an industrial electric motor. Soft starts are designed to reduce current spikes, and can offer substantial cost savings for large electric powered equipment operators.
Mechanical systems used to reduce inrush currents can include clutches and couplings which transfer or limit torque to the motor drivetrain. Electrical soft starters limit or reduce motor torque by temporarily reducing the voltage or current input. Electrical soft starters can also temporarily alter the manner in which a motor is connected to an electric network or circuit.
Soft starts, however, have several drawbacks. Typical mechanical systems are integrated in the motor power train, and can render the entire motor inoperable in the event of a malfunction. Repairing these mechanical systems can also require tools and equipment not readily available at a worksite, and equipment worksite conditions can render such repairs time-consuming or impracticable.
Electric motor soft starts are often one of the most expensive individual components of large electric-powered equipment, second only to the large motors. Electric soft starts can be complicated to troubleshoot in the event that they experience problems. Further, because electric soft starts are integrated with the other switchgear in a main electric panel, maintenance can require certified electricians or other specialized repair personal who often are not readily available. Due to the complexity of electric soft starts and device constructions that typically do not use common interchangeable parts widely available through local distributers, maintenance can be expensive and involve extended downtime. In some cases, the cost of soft start troubleshooting and repair, in addition to lost profits during downtime, can exceed any avoided current spike charges, thereby defeating the entire purpose of a soft start. In another embodiment, a variable frequency drive—whose speed may be selectively controller—may be utilized to limit current spikes. However, variable frequency drives add additional cost to the overall system beyond that of a typical motor, and require additional complexity to control and implement.
Therefore, it would be beneficial to develop a system for reducing inrush current spikes without incurring the high cost associated with prior art systems.